1. Field of the Invention
The present invention relates to the deposition or removal of material to or from a workpiece, respectively, and, more particularly, to an apparatus and method for shielding a workpiece holding mechanism from depreciative effects during workpiece processing.
2. Description of the Prior Art
In one particular application wherein the present invention is especially practical, a silicon-on-insulator (SOI) semiconductor wafer, or some other variety of substrate material such as gallium arsenide, germanium, fused silica, quartz, silicon carbide, or diamond, is undergoing a chemical etching process. During this process, a chemical etching instrument induces a removal of material from a surface of the SOI semiconductor wafer. The material is removed as a result of a chemical reaction that occurs between the surface material of the SOI wafer and a plasma that is generated by the chemical etching instrument. Such an instrument is described in a related patent application entitled, System for Removing Material from a Wafer, U.S. Pat. Application Ser. No. 7/696,897, filed on May 7, 1991. As described in this system, a chemical etching instrument probe is scanned over the surface of a wafer in a controlled pattern. While scanning the wafer surface, the probe emits a plasma that induces a material removing chemical reaction. The scanning technique that is use in this system is commonly referred to as a boustrophedon type scan, or an XY bidirectional rastor scan.
A boustrophedon type scan operates in two orthogonal directions; i.e., the X and Y coordinate planes. In the material removal system of the above related patent application, the boustrophedon type scan of the chemical etching instrument operates by scanning the probe across the surface of the wafer in a positive and a negative X-coordinate plane direction, while indexing in a positive or a negative Y-coordinate plane direction. For example, the probe may scan across the wafer surface in the positive X-coordinate plane direction, index a predetermined distance in the positive Y-coordinate plane direction, scan across the wafer surface in the negative X-coordinate plane direction, and then index a further predetermined distance in the positive Y-coordinate plane direction. This scan pattern is repeated until the entire surface of the wafer is scanned.
Although the boustrophedon type scan permits the material removal system to accomplish its basic objective of scanning the entire wafer surface, the boustrophedon method results in a depreciative etching of wafer retention materials. These wafer retention materials typically consist of a surround, upon which the wafer is placed, and several tooled registration surfaces. The surround is secured to a wafer retention stage and the tooled registration surfaces axe secured to the surround. The wafer is registered to the surround, and hence to the wafer retention stage, by the tooled registration surfaces. Furthermore, the surround and the tooled registration surfaces axe typically the same material type as the wafer to ensure a consistent chemical reaction between the surface material of the wafer and the plasma.
The depreciative etching of the surround and the tooled registration surfaces occurs when the plasma emitting probe is extended outside the edge of the wafer during the indexing steps. This depreciative etching results in damage to the wafer retention materials. Understandably, these damaged wafer retention materials must be periodically replaced resulting in an additional expenditure of capital and labor.
As described above, the use of the boustrophedon type scanning method in a chemical etching process results in depreciated material replacement problems. It is therefore desirable to use a wafer surface scan scheme that does not result in depreciated material replacement problems in a chemical etching process.